We have partially purified and characterized a human recombinase activity from RPMI 1788 B lymphoblasts. Stoichiometric amounts of recombinase carry out a strand transfer reaction between linear duplex DNA and a homologous circular single-strand DNA. The product of this strand transfer reaction is a joint molecule composed of a single-strand circle joined to one end of the linear duplex by a region of DNA heteroduplex at least 150 bp long. Formation of DNA heteroduplexes is accompanied by strand displacement. Strand invasion initiates at the ends of the linear duplex. Finally, strand displacement by human recombinase exhibits polarity and proceeds in a 3' to 5' direction. This is the first demonstration of a strand transfer activity from a high eukaryote. More recently we have pursued the purification through several columns to the point where most contaminating enzymatic activities have been removed. In addition, we have partially characterized and purified a similar protein from early embryos of Drosophila melanogaster. Finally, we have investigated the role of this lymphoblast-derived recombinase in immunoglobulin rearrangements. The proper double-strand and single-strand substrates were prepared from synthetic oligonucleotides containing the D and J recombination signal sequences. Preliminary data indicate that these D and J regions can participate in a strand exchange or transfer reaction in vitro.